The present invention relates to data highway systems for providing communications channels for distributed electronic process control systems having a number of widely spaced control stations interactively communicating in the control of a complex industrial process. More particularly, this invention relates to apparatus for connecting a large number of microprocessor based stations which are spaced over distances approaching 20,000 ft.
The control of complex industrial processes has evolved from the use of a large number of simple single loop controllers, which either perform without central direction or, alternatively, are directed by a central computer, to the use of distributed systems. In distributed systems, widely spaced control stations are connected for communication with one another and with a host computer, and each of the stations may be capable of controlling a large number of loops. The individual stations are usually microprocessor based and a host computer is employed for complex computing, control, and storage functions beyond the capability of the stations.
In complex data communication systems, such as the interacting system of the present invention, the question arises as to the type of communication channels which will provide the necessary communications between the existing stations without incordinate cost.
Electrical transmission lines have been used for effecting information transfers, but they suffer from limited bandwidth, are subject to radio frequency interference, and can present a safety hazard. Fiber optic transmission lines do not suffer from the deficiencies cited for electrical lines, but their use in sequential multi-station applications has been limited due to their cost and design constraints imposed by the difficulty of tapping fiber cables.
The disadvantages of both electrical and fiber optic transmission lines can be overcome by using a hybrid system wherein stations geographically close, as when associated with a particular part of an industrial process, are grouped by being interconnected electrically with the groups being interconnected by optical fibers.
The interface between electrical and optical lines is provided in the retransmitting units which are serially connected in the optical lines with the interface being controlled so that retransmission is stopped at the interface for the group which includes the transmitting station. The connections at the interface allow the message transmitted over the optical lines to be received by the transmitting station so that a check on the continuity of the optical and electrical lines can be made.
Bidirectional optical lines and duplicate electrical lines are provided to give fault tolerant service.